The overall frequency of p53 mutation in breast cancer is much lower compared to other cancers despite a high risk of breast cancer associated with p53 germ-line mutations. This implicates the importance of the inactivation of wild type p53 regulation in breast cancer and mechanisms other than mutation that inactivate p53 need to be characterized. Recently, it was found that BRCA1 is required for maintenance of high p53 acetylation in response to DNA damage in breast cancer cells (preliminary data). As p53 acetylation has been linked to cell senescence, the proposed study is intended to test the role of BRCA1 in p5S-mediated cell senescence in breast cancer. Hypothesis: BRCA1 may induce cell senescence through modulation of p3OO-dependent p53 acetylation in breast cancer cells. This study will examine whether BRCA1-mediated p53 activation is perturbed through disruption of p53 acetylation. Mounting data support a role for p53 in mammary tumorigenesis but the role of p53 in BRCA1-mediated carcinogenesis is still unclear. The long-term objectives are to identify novel relationships between p53 and BRCA1 in breast cancer which will improve our understanding of the pathogenesis of breast cancer, potentially revealing new therapeutic targets to design more specific and effective therapy. Specific Aims: (1) Determine the effect of BRCA1 on p53-p300 interaction and on p53 acetylation. (2) Define the role of the modulation of p53 acetylation by BRCA1 in p53-dependent transcription. (3) Determine the function of the modulation of p53 acetylation by BRCA1 in cell senescence. Study design: RNAi and overexpression studies will be used to characterize how BRCA1 affect p5S-p300 interaction and p53 acetylation in breast cancer cells. ChIP assays and transcriptional activation assays on p21/bax promoter in the presence/absence of p53, p3OO or BRCA1 will be used to dissect the functional connections between those proteins and to determine the sequence of events leading to p53-mediated transcriptional activation. Lastly, RNAi and overexpression approaches will be used to characterize p53-mediated cell senescence induced by BRCA1 using beta- galactosidase staining which is a marker of senescence. PUBLIC HEALTH RELEVANCE: This study is directly relevant to cancer as both p53 and BRCA1 are important tumor suppressors. This study is unique because most studies focus on mutations in tumor suppressor genes while it points out the importance of wild-type p53 regulation in breast cancer and define a novel pathway other than mutations that lead to p53 inactivation. These novel links between BRCA1 and p53 will help identify those individuals at increased breast cancer risk and to devise more specific therapies.